A. Field of the Invention
This invention is related to an apparatus and a method for determining the optical distortion of a transparent substrate, such as a float glass sheet, as a function of the changes in the separation of two coherent light beams reflected by the surfaces of the glass sheet, by means of a methodology of real time digital image analysis.
B. Description of the Related Art
In the manufacture of transparent sheets or plates, such as in the manufacture of glass sheets by the floating process, the drawing process or other processes, some defects may appear in the resulting glass sheets.
The optical distortion of the glass sheet is among the above mentioned defects. One type of optical distortion is the distortion of the glass surfaces which produces a distorted reflection of an image such as, for example, convex portions of the glass surface produce a shrinkage of the reflected image while concave portions produce a magnifying of the image.
Another kind of distortion is the transmitted distortion which produces a distortion of the image when it is seen through the glass.
In U.S. Pat. No. 3,788,750 of Maltby et al, issued on Jan. 29, 1974, the optical distortion, or else the optical quality of the glass, is measured regarding the power of the focal length of a lens measured in diopters which are defined as the unit over the focal length expressed in meters.
In the U.S. Pat. No. 5,210,592 of Bretschneider, issued on May 11, 1993, the optical quality is determined as the optical power defined as the shunt of the angle observed, for example the angle of reflection or the angle of transmission after a location. The greater the local curvature of the surface of the glass, the greater will be the optical power.
A method for describing the distortion of a surface of a glass piece is disclosed in the U.S. Pat. No. 3,857,637 of Obenreder, issued on Dec. 31, 1974. This patent also discloses an apparatus for determining the optical power reflected on an upper surface of a glass piece, which requires, in some cases, a light absorbent material, for example a black painting in optical contact with the lower surface of the glass piece to prevent any reflection of the light beam by this surface. This is for glass sheets of a thickness less than 3/32 of an inch and an incidence angle of 30.degree.. For thickness greater than 3/32 of an inch, a "shell" provided by the apparatus can be suitably used. It is well known that this optical power can be measured for both surfaces if the glass piece is turned down and measured again from the rear surface, but the distortion of both surfaces of the glass sheet cannot be determined in a single operation.
U.S. Pat. No. 4,585,343 of Schave et al, issued Apr. 26, 1986, discloses an apparatus for detecting the distortion of a surface; the apparatus includes a source of light mounted to direct a first light beam toward a surface of the glass sheet at an oblique incidence angle to cause a second light beam when it is reflected by said surface; a light detector mechanism is mounted to receive the reflected light beam which is sensitive to a light pattern on the detector mechanism, produced from the reflected light beam generating an output signal representing the width of the light of said pattern. In this case, the width of the light of the pattern is a function of the surface distortion of the portion of the surface from which the light beam is reflected. While the apparatus is capable of determining the distortion of the surface, this does not represent said distortion in terms of optical power.
The U.S. Pat. No. 5,210,592 of Bretschneider discloses an apparatus in which two parallel light beams having a reciprocal space are directed toward a plate under an acute angle regarding the normal of the plate, the light beams reflected by said plate are received separately by a detector mechanism with a photosensor device and the direction of the light beams reflected is evaluated, in which the four light beams reflected by the two surfaces of the plate over the detector sensitive to the position are necessary to evaluate the parameters for the quantitative determination of the optical quality of the plate. The apparatus is well accepted in terms of the determination of the optical power.
In the present invention it is disclosed that, in order to determine the optical power of a transparent plate, the use of mechanisms for the separation of the light beams, as it is done in the apparatus of the patent of Bretchneider, is not necessary and that the four reflected light beams are not required, but just two of them are enough to determine the optical power by means of the introduction of technique of real-time digital image analysis. This change involves mechanisms different from those disclosed by Bretchneider.
The apparatus of the present invention includes a light source mounted to direct a first light beam toward a surface of the plate at an incidence angle, based on the normal, between 44.degree. and 54.degree., preferably of about 49.degree., to cause a second light beam which is reflected specularly to the upper surface of the plate, and a third light beam reflected from the lower surface thereof. The separating space between the light beams is a function of the incidence angle, of the refraction index of the material and of the thickness thereof.
A device is provided for the integration of the reflected light beams, by means of a rotary screen of a diffuse material to obtain two perfectly defined spots from the light beams. Devices are provided for sensing the relative position of the light beams specularly reflected from the surfaces when the plate is moved along a predetermined path with known speed. Calculation means, based on the technique of the real-time image digital analysis, are used to measure the separation of the light beams and computer calculation means are also provided for the determination of the distortion in terms of the optical power, whose internationally-recognized measure unit is the diopter.
As the light beam is swept along the plate, the separation space of the reflected light beams changes in an inverse proportion to the focal distance of the lenses which are formed in the glass (concave or convex). By calculation means involving the use of the real-time digital image analysis technique, the determination of the superficial and transmitting optical distortion is carried out.
A resulting output graphic is provided wherein the longitudinal axis represents the position along the sample, while the vertical axis represents the optical power of the glass. The extent of the curves is proportional to the optical power of the glass, consequently the graphic indicates the distortion in terms of the optical power in any point along the length of the glass sample.
As can be observed, the light source can be displaced in a relative way to the material, or the material and the light source can be moved in turn one in relation to the other.